This invention relates to co-substituted linear polyphosphazene polymers and to their use in gel polymer electrolytes. This invention relates also to the preparation of co-substituted linear polyphosphazene polymers and to the preparation of gel polymer electrolytes in which the linear polyphosphazene polymers of this invention are an essential component.
Gel electrolytes are alternatives to both solid polymer electrolytes (hereinafter, SPEs) and liquid electrolytes for battery applications. Gel electrolytes possess some of the advantages and disadvantages of both the solid and liquid systems. In organic, liquid-filled batteries, the liquid electrolytes may escape or present a fire hazard and an inert spacer generally is needed to separate the electrodes. Solid polymer electrolyte systems typically possess the mechanical properties and structural integrity required for battery applications, but have inherently lower conductivities due to the more restricted motion of the polymer molecules. Solid polymer electrolytes are non-volatile, non-corrosive materials, which can readily be processed into virtually any shape or size. In addition, the inherent lightness of weight and flexibility of solid polymer systems enable the production of more robust energy storage devices having high energy densities.
Solid polymeric systems based on poly(ethylene oxide) (hereinafter, PEO) have been investigated thoroughly due to their inherent mechanical advantages over conventional liquid based batteries (see, for example, Armand, M. B., et al, Second International Conference on Solid Electrolytes, Armand, M. B., et al Ed., Andrews, Scotland, 1978; Wright, P. U., et al, Polymer, 14, 589 (1973); Gray, F. M., Solid Polymer Electrolytes: Fundamentals and Technological Applications; VCH Publishers, Inc., New York (1991); and Vincent, C. A., et al, Polymer Electrolyte Reviews; Vincent, C. A., et al Ed., Elsevier Applied Science, New York Vol 1 and 2 (1987). However, despite the benefits afforded by solid polymer electrolytes, the maximum ambient temperature ionic conductivity achieved to date is in the range of xcx9c5xc3x9710xe2x88x925 S/cm. (Gray, F. M., Solid Polymer Electrolytes: Fundamentals and Technological Applications; VCH Publishers, Inc., New York (1991))
Gel polymer electrolyte systems are an attempt to strike a balance between the high conductivity of organic liquid electrolytes and the dimensional stability of solid polymer electrolytes. Gel systems can reach the commercially desired conductivity of 10xe2x88x923 S/cm, but typically only when large amounts of an organic liquid are present. Thus, they may suffer from the same problems as liquid electrolytes (see, for example, Sung, H, et al, Journal of the Electrochemical Society, 145, 1207 (1998); Croce, F., et al, Electrochemica Acta, 39, 2187 (1994); Ballard, G. D. H., et al, Macromolecules, 23, 1256 (1990); and Allcock, H. R., et al, Macromolecules, 30, 3184 (1997)).
A goal in the synthesis of gel electrolyte systems is to produce a dimensionally stable gel, which can attain high levels of ionic conductivity with minimum amounts of organic additives. The design of such gel systems depends on an understanding of the mechanism of ionic conduction in gels, and on the ability to tune the structure of the polymer component in the gel to optimize the overall physical properties.
In solid poly(ethylene oxide) and related systems that are complexed with a metal salt, such as a lithium salt (i.e., most SPEs), lithium ions coordinate with the oxygen units in the etheric chains, and Li+ ions are passed from one chain segment to another as the polymer undergoes reptation and side chain reorientation (see, Allcock, H. R., et al, Contemporary Polymer Chemistry, 2ed., Prentice Hall, Englewood Cliffs, N. J. (1990); Bruce, P. G., et al, Journal of the Chemical Society: Faraday Transactions, 89,3187 (1993); and Gray, F. M., Polymer Electrolytes, The Royal Society of Chemistry, Cambridge, U.K. (1997)). In order to maximize the transport of ions through the matrix, the polymer must be completely amorphous and have a low glass transition temperature (Tg) to facilitate motion of the polymer chains (Meyer, W. H., Advanced Materials, 10, 439 (1998)).
In organic liquid electrolytes, the Li+ ions are surrounded by coordinative solvent molecules and migrate through the liquid via diffusion (Armand, M., Advanced Materials, 2,278 (1990)). In gel electrolytes, both mechanisms are possible (i.e., diffusion of organic liquids and molecular reorientation by polymer chains and their side groups), although a solvent-assisted mechanism on first consideration appears to be more plausible. However, the extent to which each process affects the ionic conductivity of gel electrolytes is not well understood. One view is that the liquid component plays only a minor role in the movement of the ions, and serves mainly as a plasticizer (Gray, F. M., Solid Polymer Electrolytes: Fundamentals and Technological Applications; VCH Publishers, Inc., New York (1991)). In this interpretation the liquid functions mainly to increase the free volume and lower the Tg to allow more facile movement of polymer chains and ions. In this case, the pathway for ionic conduction would mainly involve the polymer and its molecular motion. A second theory suggests that the liquid forms miniature xe2x80x9cchannelsxe2x80x9d within the polymer through which solvated ions can move freely via diffusion: In this case, the polymer would serve only as a supportive matrix (Koksbang, R., et al, Solid State Ionics, 69, 320 (1994)). The mechanism of conduction may he somewhere between these two extremes and, almost certainly, would depend on the specific system.
Poly(acrylonitrile) and poly(methyl methacrylate) based systems, for example, have been studied as the polymeric component in gel electrolytes. The poly(methyl methacrylate) studies support a mechanism of ionic conductivity controlled primarily by the diffusion of small molecules through the polymer matrix. However, the ionic conductivity of the poly(methyl methacrylate) systems ultimately is supplemented by the faster segmental motion of the polymer backbone due to plasticization (Svanberg, C., et al, Journal of Chemical Physics, 111, 1 1216 (1999)). Poly(acrylonitrile) systems, on the other hand, provide clear evidence against the formation of miniature xe2x80x9cchannelsxe2x80x9d of liquid since the ionic mobility is impeded more in a gel system than in a liquid system (Stallworth, P. E., et al, Solid State Ionics, 73, 119 (1994) and Edmondson, C. A., et al, Solid State Ionics, 85, 173 (1996)). Moreover, the measurement of dielectric constants indicates a probable interaction of the metal salt component with the polymer component, which is a further indication of participation by the polymer in the conduction mechanism of the gel electrolytes (Stallworth, P. E., et al, Solid State Ionics, 73, 119 (1994)).
A system that has been studied extensively for SPE applications is one that is based on poly(organophosphazenes). This class of polymers has yielded excellent candidates for use in SPEs due to the inherent flexibility of the phosphorus-nitrogen backbone and the ease of side group modification via macromolecular substitution-type syntheses. The first poly(organophosphazene) to be used in a phosphazene SPE was poly[bis(2-(2xe2x80x2-methoxyethoxy ethoxy)phosphazene] (hereinafter, MEEP). This polymer was developed in 1983 by Shriver, Allcock and their coworkers (Blonsky, P. M., et al, Journal of the American Chemical Society, 106, 6854 (1983)) and is illustrated in FIG. 1. This polymer showed ambient temperature conductivities of xcx9c10xe2x88x925 S/cm, which is several orders of magnitude higher than that of solid PEO. Although the development of MEEP was a significant breakthrough in SPE research, its gum-like character requires that it be crosslinked before it can be processed as a free-standing film for energy storage applications.
In addition to MEEP, many other polyphosphazenes have been examined as SPEs, including polymers having longer ethyleneoxy chains, polymers having branched structures, and polymers having crown ether-containing side groups. These polymers were examined, at least in part, in an attempt to improve both the mechanical stability and ionic conductivity of SPEs (see, for example, Allcock, H. R., et al, Macromolecules, 30, 3184 (1997); Allcock, H. R., et al, Macromolecules, 29, 1951 (1996); Ailcock, H. R., et al, Macromolecules, 29, 3384 (1996); Allcock, H. R., et al, Macromolecules, 29, 7544 (1996); and Allcock, H. R., et al, Macromolecules, 31, 8036 (1998)). These modifications significantly improved the mechanical stability of the polymers, but the maximum conductivity was increased only slightly to xcx9c5xc3x9710xe2x88x925 S/cm.
Accordingly, there remains a need for developing still further polymers that possess enhanced mechanical stability and an ability, when used in electrolyte systems, to improve the ionic conductivity of the electrolyte systems relative to the ionic conductivity exhibited by SPEs known in the art.
It is an object of the present invention to provide li near phosphazene polymers that are characterized by mechanical stability and suitability for use in gel polymer electrolytes SPEs.
Another object of the invention is to provide co-substituted linear phosphazene polymers that contain side chains that promote ionic conductivity, as well as side chains that increase mechanical stability.
Still another object is to provide MEEP-type polymers containing methoxyethoxyethoxy side chains to promote ionic conductivity and non-conductive hydrophobic fluorine-containing side chains to increase mechanical stability.
Another object is to provide gel polymer electrolyte systems that comprise (i) co-substituted linear phosphazene polymers that contain side chains that enhance the mechanical stability of the polymer and electrolyte systems, (ii) small organic molecules that promote ionic conductivity of the electrolyte systems, and (iii) a metallic salt that promotes ionic conductivity of the electrolyte systems.
Another object is to provide gel polymer electrolyte which contain co-subsituted phosphazene polymers as the only essential polymer component and which exhibit a conductance at room temperature of at least about 10xe2x88x925 S/cm, and preferably, at least about 10xe2x88x924 S/cm.
Yet another object is to provide gel polymer electrolyte systems that are based on a MEEP-type linear phosphazene polymer that contains methoxyethoxyethoxy side chains to promote ionic conductivity and non-conductive, hydrophobic fluorinated side chains to increase mechanical stability of the systems.